1. Field of the Invention
This invention relates to mid-IR applications. More specifically, this invention relates to sapphire panes and a method of bonding sapphire panes together in order to make a larger, monolithic sapphire structure which can be used in mid-IR applications.
2. Brief Description of the Related Art
Sapphire, a single crystal form of aluminum oxide (Al.sub.2 O.sub.3), is commonly used in electron tubes, microwave tubes, and aluminum composites. Sapphire is also used as optical elements in radiation detectors, as substrates for thin-film components in integrated circuits, and in other precision instruments.
Sapphire has a structural integrity and temperature stability suitable for many military applications and is able to transmit light in the visible, infra-red (IR), and ultra-violet (UV) spectra. Sapphire is typically found in the form of a rod, a whisker, a single crystal, a sphere, or a small disc. Because of its structural integrity, temperature stability, and excellent light transmission capabilities, sapphire would be ideal for use as an optical element in military optical sensor equipment. Its use, however, is limited due to the availability of large size parts.
Heretofore, it has not been possible to manufacture a larger, monolithic sapphire structure by bonding together smaller sapphire elements (or panes) without compromising the superior light transmission properties of the individual sapphire panes. Bonding technology known in the art (e.g., mechanical attachment, adhesive bonding, glazing, brazing, diffusion bonding, and/or fusion welding) generally has not proven to be suitable for joining individual sapphire panes together.
For example, diffusion-bonding (or fusion-welding) of two substrates to each other typically is accomplished by a solid-state reaction using high pressures and temperatures. While diffusion bonding similar substrates together usually produces a contact between the substrates, the bond may be of a low structural integrity. In order to overcome the possible structural shortcoming, an interlayer bonding material diffused between the substrates at the bond interface is often used to form a stronger bond. Since sapphire is a single crystal material, the interlayer bonding material must maintain epitaxy between the joined sapphire pieces. Any thermal expansion mismatch between the substrate and the interlayer bonding material must be minimized to avoid the formation of expansion stress cracks. Alternatively, the conditions for joining must be selected to provide a thin bond line that will not crack in response to a thermal expansion mismatch.
While use of the interlayer bonding material may satisfy the structural integrity requirement demanded of sapphire in military environments, the interlayer bonding material must also satisfy the optical requirements demanded in mid-IR applications. The interlayer bonding material and area of bonding at the interface of the individual sapphire panes must be able to transmit light in the visible and mid-IR spectra. This requirement for a mid-IR material sharply reduces the number of materials available for diffusion bonding sapphire panes.
Other methods for bonding together various ceramic materials include bonding at low temperature with cementitous materials or bonding at elevated temperatures with the use of metalizers. Still another method uses a eutectic mixture of aluminum oxide and zirconium oxide to form a eutectic bond between the sapphire elements. However, these methods use bonding materials (e.g., metal, silicon oxide, and/or zirconium oxide) whose crystallography and optical properties do not provide a satisfactory epitaxy or mid-IR light transmission at the bond interface.
It would be desirable to provide a method of making a large, monolithic sapphire structure from individual sapphire panes, wherein the structure has approximately the same mechanical, optical, and crystallographic properties of the individual panes. Furthermore, it would be desirable to provide a method of making a large, monolithic sapphire structure having a structural integrity and optical characteristics suitable for use in mid-IR applications.